"Mr. Romney is one of those people who only takes his foot out of his mouth to change feet," Rep. John Dingell, D-Mich., said with a top United Auto Workers official at the Detroit-Hamtramck Assembly plant where the Volt is made.
"He says the Volt, which has had its best month of sales yet, is an idea whose time has not come," Dingell said.
Romney, who grew up in Michigan as the son of George Romney -- the governor of Michigan and chairman and president of now-defunct American Motors Corp. -- took his second shot at the Volt in four months Tuesday while campaigning in Wisconsin on the day of the Wisconsin primary he won.
"I'm not sure America was ready for the Chevy Volt," he said. "I mean, I hope it does well -- I don't want to disparage any product coming out of Detroit. But I think instead of having politicians tell us what kind of cars we ought to make, we ought to let the people who are trying to understand the market make that decision."
Romney laughingly dismissed the Volt on a Boston radio program in December as "an idea whose time has not come."
Detroit's Big Three automakers said Tuesday they had stronger-than-expected sales in March, driven partly by high-efficiency vehicle sales, including the Volt.
The U.S. Environmental Protection Agency says the Volt is the most fuel-efficient compact car with a gasoline engine sold in the United States.
While the car -- sold as the Opel/Vauxhall Ampera in Europe and the Holden Volt in Australia -- has won awards, some conservative politicians have raised questions about the extent of the federal government participation in the Volt development during General Motors' 2009 government-led bankruptcy.
President Barack Obama told a UAW conference Feb. 28 he intended to buy a Volt when he leaves office.
"I got to get inside a brand-new Chevy Volt fresh off the line -- even though Secret Service wouldn't let me drive it," he said.
"But I liked sitting in it. It was nice. I'll bet it drives real good," he said. "And five years from now, when I'm not president any more, I'll buy one and drive it myself. Yes, that's right."
The Volt was introduced as a concept vehicle in January 2007. Tax incentives for purchasing high-efficiency and plug-in vehicles like the Volt were passed under President George W. Bush.
General Motors Co. confirmed Wednesday it plans to resume production of the Volt April 16, one week before production had been scheduled to resume, because sales of the model are picking up, the Detroit Free Press reported.
"Employees are being called back one week earlier than previously announced due to increased Volt sales since January and the need to meet demand in our strong markets, including California," GM said in a statement.
GM said Tuesday it sold 2,289 Volts in March -- a 50 percent jump in sales from December, when Volt sales set a monthly record -- the Free Press reported.
"We are meeting supply and demand," Mark Reuss, president of GM North America, said. "It's not just Volt. It's everything."